U.S. Pat. No. 4,759,182 and JP 2004-196110A published by the Japan Patent Office in 2004 disclose an oil pressure supply circuit which drives a power steering device and a cargo handling device of a fork-lift truck using a single oil pump.
In both prior art circuits, a priority flow rate control valve is provided for distributing pressurized oil from the oil pump to the power steering device preferentially and distributing excess oil to the cargo handling device. For this purpose, a load pressure on the power steering device is caused to act on a valve spool of the priority flow rate control valve via a pilot passage as a pilot pressure and a distribution ratio of pressurized oil to the power steering device is increased as the pilot pressure increases.
The pilot pressure is introduced to the pilot passage via an orifice. An oil pressure supplied to the power steering device is also introduced to the pilot passage via another orifice. The valve spool is supported elastically by a spring A in the same direction as the pilot pressure acting on the valve spool. On the other hand, the oil pressure supplied to the power steering device acts on the valve spool in the opposite direction to the pilot pressure. The valve spool distributes pressurized oil to the power steering device and the cargo handling device in a distribution ratio determined according to the balance of forces acting on the valve spool.